The question asks for a major health mission carried out in 1996.

Option 1: War against Leprosy - This is incorrect. While efforts to combat leprosy are ongoing, it didn`t specifically begin in 1996.

Option 2: Small-pox eradication - This is also incorrect. The World Health Organization (WHO) officially declared smallpox eradicated in 1980, so it`s not relevant to the 1996 timeline.

Option 3: Pulse Polio Immunization - This is the correct answer. The Pulse Polio Immunization program was initiated by the government of India in 1995-1996 to eradicate polio in India. This involved mass immunization campaigns several times a year to ensure every child under the age of five was protected.

Option 4: Popularisation of oral rehydration therapy - Incorrect option as the WHO and UNICEF issued a joint statement promoting oral rehydration therapy as early as 1978, long before 1996.

Hence, the correct answer is Option 3: Pulse Polio Immunization.